Thermoluminescent radiation detector

ABSTRACT

Radiation detector having a spring clip holding hot-pressed thermoluminescent chips in contact with a heater element welded to electrical leads passing through a glass envelope enclosing the same.

United States Patent [191 Winn et al.

[ Nov. 27, 1973 THERMOLUMINESCENT RADIATIO DETECTOR Inventors: Ray Winn,33 Elm Street,

Wakefield, Mass. 01880; Joseph C. Ennis, 57 Topsfield Road. Ipswich,Mass. 01938 Filed: Dec. 14, 1970 Appl. No.: 97,685

vs. C: 250/71 R 1m. 01. G0lt 1/11 Field of Search 250/71 R, 71.5 R

References Cited UNITED STATES PATENTS 4/1968 Rutland ct a1. 250/71 R X3,419,720 12/1968 Debye et al 250/71 R X 3,555,277 1/1971 Attix 250/71 R3,590,245 6/1971 Oonishi et al 250/71 R Primary ExaminerWilliam F.Lindquist Assistant Examiner-Davis L. Willis Attorney-J. Herman Yount,Jr.

Sundheim and Robert B.

[5 7] ABSTRACT Radiation detector having a spring clip holdinghotpressed thermoluminescent chips in Contact with a heater elementwelded to electrical leads passing through a glass envelope enclosingthe same.

16 Claims, 7 Drawing Figures l THERMOLUMINESCENT RADIATION DETECTORBACKGROUND The present invention relates to apparatus for-measuringionizing radiation, and more particularly, toanv improvedthermoluminescent radiationdetector.

Exposure to ionizing radiation, such as xrays, gamma rays, cosmic rays,and nuclear radiation generally, constitutes a serioushazard to humanbeings. Moreove, activities involving exposure to ionizing radiation areincreasing. For example, medical and dental practitioners expose theirpatients and-themselves to x-rays and gamma ray radiation whileperforming. a variety of diagnostic and therapeutic procedures. Nuclearfission atomic power plants, both stationary and moage, such personsmust monitor the total. radiationwhich their bodies receive. In.addition, there-arenumerous other requirements inindustryand inresearchlaboratories for monitoring radiation'with highreliability. To do this,a variety of instruments, such'as ionizing chambers, Geiger counters,scintillation detectors, and

others, are used. These require power tooperate, andhave otherdisadvantages insofar as personnel radiation monitoring is concerned.Heretofore, passive or nonpowered dosimeters, which integrate or sum thetotal incident radiation, have proven most useful-for'persom nelmonitoring. Most widely used are the small pocketsized electroscope andthe photographic film badge.

The photographic film badge dosimeter requires considerable processingafter exposure to developthe film and to translate the developed filminto units of radiation dosage. Then it is reloaded with fresh unexposedfilm before it is used again. Photographic film badge dosimeters arereliable through the dosage range in which they are sensitive; however,they. lack as wide a dosage range as is desirable.- Also, they lackclose tolerance and quantitative reproducibility. Further, processingfor readout is too complicated to be practicalin field or disasterenvironments. Thus, there are good reasons for replacingthe photographicfilm badge'dosimeter.

Another pocket-sized, easily read dosimeter utilizes the electroscope.These are easy to charge and to calibrate under field conditions and areconveniently read visually. However, they are highly sensitive tomechanical shock and rough handling, both of which cause loss ofcalibration. Therefore, their reliability is always questionable. Theydo have the advantage of not requiring complex processes for. readoutand reloading prior to reuse, but may be readout visually andrecalibrated by very simple means.

Thus, the need exists for a highly reliableradiation dosimeter which maybe used over and over again without additional reloading, complexprocessing, or calibration. Such a dosimeter must have a .very highreli-' ability and good accuracy under the most severe of fieldconditions or disaster environments. This need has led to serious andexpensive efforts to adapt the phenomenon of thermoluminescence topersonnel radiation dosimetry.

Thermoluminescence is a phenomenon observed in a number of materials,some of which occur naturally, in which electrons are sufficentlyexcited by impinging ionizing radiation to undergo transitions tocertain metastable states or traps. From there they may be excited byheat energy to undergo further transitions to emitting-states from whichthey experience optical transitions back to the ground state, emittingvisible light during these latter transitions.

Thermoluminescent materials can now be prepared 7 which exhibit goodreproducibility in their response to radiation dosage. Further, theymaybe exposed repeatedly, even hundreds of times, to radiation, eachradiation exposure being quantitatively impressed upon the material andthey may be quantitatively read out upon heating between each exposure.Despite extensive reuse, the response of such samples ofthermoluminescent materials to ionizing radiation remains unchanged.

To determine the amount of exposure, the thermoluminescent material isheated up to about 300 C during which it luminesce's. The recording ofluminescent brightness versus temperature taken at a constant heatingrate is called the glow. curve. The numberof different types of traps inthe material and the energy by which the electrons are bound in thesetraps determine the number of peaks in the glow curve and thetemperature at such peaks. With shallow traps (less binding energy)moderate ambient temperatures release the trapped electrons and visiblephotons. The'deeper the trap, the higher is the glow peak temperature,and the more stable is the thermoluminescent signal of the phosphor atambient-temperatures. The thermoluminescent brightness for a givenexposure depends on the concentrationof trapping sites (quantumefficiency) and on the efficiency of the transitions back to the groundstate. The rate of heating the phosphor also affects the glow curve,although the total light emitted is the same regardless of heating rate.Faster heating gives narrower glow curves of higher peak brightness andshifts the peak emission to higher temperatures.

When used in a personnel dosimeter the thermoluminescent detector isenclosed in a radiation shield to achieve uniformity of response toionizing radiations of various energies. During readout light sensitiveapparatus detects the luminescent output of the thermoluminescentmaterial during the heating process and converts it to an electricalsignal. Recording apparatus may be utilized to record the entire glowcurve, including the peaks. Either the area under the glow curve or aportion thereof, or the brightness of emitted light at the maximum glowpeak constitutes a measure of ionizing radiation dosage. Alternatively,the maximum glow peak may be measured electronically and converted toand visible photons are not emitted at normal ambient temperatures. Anymaterial having an appreciable number of shallow and intermediate depthtraps, which are depopulated at ambient and moderately elevatedtemperatures with the passage of time, precludes its use for dosimetry.

Earlier endeavours to construct practical personnel radiation dosimeterswith thermoluminescent phosphor material were not successful becausetheir detectors were insensitive to low dosage rates, or were unstableand released trapped electrons spontaneously at ambient temperatureswith the passage of time. Continued efforts to develop thermoluminescentmaterials suitable for dosimetry resulted in the production ofmanganese-activated calcium fluoride, which contains deep traps almostexclusively. One serious disadvantage of the manganese-activated calciumfluoride is its undesirable chemical activity during processing. Otherdeeptrap thermoluminescent materials occur in nature in limitedquantities and can be manufactured. These include lithium fluoride,calcium sulphate, and some organic materials.

Various schemes have been devised and proposed for the use ofthermoluminescent materials in practical personnel dosimeters. One earlydevice utilized only the glow peaks of thresholds of various thermoluminescent materials confined within a glass container. No effort was madeto read the total radiation quantitatively, but the radiation dosage wasestimated to be be tween that minimum dosage that would produceluminescence in the highest threshold" material that luminesced andbelow that of the next higher threshold material that did not luminesce.Obviously, this device was not a practical dosimeter because it couldnot accurately measure radiation dosage.

A later endeavour included dosimeters prepared by mixingthermoluminescent powder with temperature resistant transparentcementing materials such as a mixture containing potassium silicate andthen coating the mixture onto heating elements enclosed within a glasstube in an inert gaseous environment of low thermal conductivity.Dosimeters constructed in this manner give reliable readings for dosagesas low as 50 mr. with little or no spurious luminescent effects. Onedifficulty encountered with this latter technique of mounting thephosphor is that repeated heating to temperatures in excess of 30 Cduring readout caused scaling and breaking up of the thermoluminescentcoatings. Another difficulty is that potassium silicate in cementingmixtures is naturally radioactive and that dosimeters containing thesame naturally accumulate dosage at the rate of about 0.4 milliroentgenper day. This is objectionable in some applications.

Another scheme contemplates compressing thermoluminescent phosphorpowder consisting of a mixture of relatively large and small granularsizes, within a container having a transparent wall. Only sufficientcompression is utilized to avoid relative motion of the powder granuleswith respect to each other and with respect to the container walls. Thisis done to avoid spurious thermoluminescence of various kinds.

In still another scheme the electrical heating element threads throughthe coaxial hole passing through a cylindrically-shaped, hot-pressedthermoluminescent member of predetermined length. A glass envelopeencloses this assembly in a gaseous environment of high thermalconductivity. A major difficulty with this scheme resides in themanufacturing of the cylindrically-shaped, hot-pressed thermoluminescentmembers. No efficient way has been found to hot-press these members tothe dimensions desired in manufacturing quantities. Thus, conventionalultrasonic drill presses using specially designed steel cutters are usedto cut these members from base pieces of hot-pressed thermoluminescentmaterial. Obviously, a large amount of this material is wasted. Worse,the rejection rate of thermoluminescent members produced by thisultrosonic means is totally unacceptable in a manufacturing process. Asecond major difficult occurs during readout. Air gaps between theheater wire and the thermoluminescent member prevent good, uniform heatconduction therebetween. Use of a sealing material between the heaterwire and the thermoluminescent member does little to improve heatconduction. Consequently, during heating, certain points on the heaterwire become hotter than others. These points are commonly called hotspots. These hot spots may radiate light in the visible spectrum orinfrared radiation. The result is that such a detector produces aslightly erroneous reading at low radiation dosages. For example, if anunirradiated detector of this type is passed through a heating cycle, itexhibits about a 0.5 mr equivalent dose. This is unacceptable in lowdosage radiation dosimetry.

Moreover, these schemes do not provide an inexpensive, practical,sensitive, mechanically rugged dosimeter.

Deep-trap thermoluminescent materials, sensitive to radiation in themilliroentgen range, may be prepared in the laboratory. These materialshave a further advantage of being linear in their response to radiationthrough as much as seven decades of radiation dosage. The desirabilityof adapting these thermoluminescent materials to personnel radiationdosimeters is clear. There remained the practical problem solved by thepresent invention of providing a thermoluminescent radiation detectorwhich is sensitive to very small radiation dosage, which may be usedover and over after repeated readout, and which will be inexpensive tomanufacture.

The present invention involves a unique thermoluminescent radiationdetector utilizing hot-pressed thermoluminescent materials madeaccording to the process described in application Ser. No. 432,804 filedFeb. 15, 1965, now US. Pat. No. 3,567,922 by Gerald E. Blair andassigned to the same assignee as the present invention.

08.! ECTS Accordingly, one object of this invention is to provide animproved an improved thermoluminescent radiation detector.

Another object of this invention is to provide an inexpensive,sensitive, and reliable thermoluminescent radiation detector which maybe accurately reproduced in great quantity.

Another object of the present invention is to eliminate any binders thatmay contain naturally radioactive materials, thus preventing naturalaccumulation of dosage.

In one type of prior art thermoluminescent radiation detector themanufacturing rejection rate averaged about percent in order to producedetectors having an acceptable sensitivity of i 20 percent. Thus, stillandetector, the thermoluminescent material was enclosed in an inertenvironment of low thermal conductivity or in an evacuated chamber. Itwas believed that this completely suppressed spurious luminescence thusproviding a threshold of practical detection of radiation dose in thelow milliroentgen range. In particular, it was believed that theconstituents of air oxygen, nitrogen and carbon dioxide gave rise tospurious luminescence, thus preventing reliable detection of radiationdoses in the milliroentgen range. It has been discovered that this isnot so. Consequently, it is still another object of the presentinvention to provide a thermoluminescent radiation detector of superioroperating characteristics in which the thermoluminescent material isenclosed in gaseous environments of high thermal conductivity such ashelium or gaseous environments of carbon dioxide.

Other objects and various further features of novelty and invention willbe pointed out or will occur to those skilled in the art from a readingof the following specification and claims.

DESCRIPTION OF THE DRAWINGS The invention is more easily described byreferring to the following illustrations in which:

FIG. 1 is a side view of one embodiment of the present invention;

FIG. 2 is a view showing construction of part of the embodiment of FIG.1; I

FIG. 3 illustrates the electrical leads mounted in the base of theembodiment of FIG. 1;

FIG. 4 illustrates the spring clip utilized to maintainthermoluminescent chips in contact with the heater element;

FIG. 5 is a sectional view showing construction of an embodimentalternative to the sub-assembly of FIG. 2; and

FIGS. 6 and 7 illustrate still other embodiments of the presentinvention.

DETAILED DESCRIPTION In U. S. Pat. No. 3,282,855, issued Nov. 1, 1966 toR. C. Palmer et al. for Method of Making ThermoluminescentManganese-Activated Calcium Fluoride" and assigned to the assignee ofthe present invention, the patentees disclose a method of makingmanganeseactivated calcium fluoride. The method comprises mixing anaqueous slurry of calcium carbonate and l to 10 mole percent ofmanganous carbonate with a concentrated solution of hydrofluoric acid.The reaction is quite vigorous and after 2 or 3 minutes a coprecipitateof calcium fluoride and manganous fluoride forms. When the coprecipitateceases forming, it is washed 3 or 4 times with either deionized ordistilled water to re- 6. move all hydrofluoric acid and by-products ofthe reaction. It is then dried at about C, producing a powdered mixtureof calcium fluoride and manganous fluo ride. At this stage thecoprecipitate is not useful as a thermoluminescent material because itis only slightly thermoluminescent. The powdered coprecipitate is thenplaced in a platinumcrucible and heated in a dry inert atmosphere for 30minutes at a temperature of about l,200 C. During this heating thecoprecipitate becomes a cake of manganese-activated calcium fluoridewhich is highly thermoluminescent. After cooling, the cake may be brokenup and pulverized into a powder for use in manufacturingthermoluminescent dosimeter's. The latter step of heating at l,200 C iscalled activating because it is believed that such heating forces manymore manganese ions into the crystal lattice of the calcium fluoridecreating many more deep traps thereby making the material highlythermoluminescent. The above identified patent to Blair discloses thatthe 1,200 C heating step and the additional detector manufacturing stepsof coating some substance with manganese-activated calcium fluoride maybe-omitted and that hot-pressing techniques may be utilized toconcurrently activate and form the coprecipitated calcium fluoride andmanganous fluoride into a solid, activated thermoluminescent material.

FIRST EMBODIMENT Referring to FIGS. 1 through 4, one embodiment of the,present invention comprises transparent glass envelope 20 that may besealed to base 22 by known techniques. Heater current leads 24 and 26pass through base 22 and are sealed thereto. Leads 24 and 26 are bent toform arms 28 and 30 respectively. Ribbon heater wire 32 is spot weldedto arms 28 and 30. Spring clip 34 maintains chips 36 and 38 in surfaceto surface contact with heater ribbon wire 32. Chips 36 and 38 arehot-pressed thermoluminescent material such as, for example,manganese-activated calcium fluoride or lithium fluoride. Afterassembly, glass envelope 20 may be evacuated to maintain the assembly invacuum or it may be filled with a gas of high thermal conductivity,prior to scaling at tip 40.

Referring now to FIG. 2, during assembly, arms 28 and 30 are pulledslightly together and ribbon heater wire 32 is stretched taut when spotwelding ribbon heater wire 32 to arms 28 and 30. This assures thatribbon heater wire 32 does not wrinkle or buckle when heated by heatercurrent during readout of radiation doses received by the detector.Chips 36 and 38 may be made on a semi-conductor wafering machine from acake of hot pressed thermoluminescent manganeseactivated calciumfluoride or from hot-pressed lithium fluoride. Such lithium may be LiLi, or natural lithium. Typically, chips 36 and 38 have dimensionsonefourth inch long by 0.070 inch wide by 0.035 inch thick. Ribbonheater wire 32 may be Stablohm 650 ribbon wire with typical dimensions0.045 inch long by 0.375 inch wide by 0.0035 inch thick. It will beunderstood that ribbon heater wire 32 may be cut to the length desiredto produce the heating desired.

With the above typical dimensions spring clip 34 originally has thetypical dimensions illustrated in FIG. 5 and may be made of 0.016 inchdiameter NISPAN C. During assembly of the above-described embodiment itis cut to a length of approximately 0.070 inch. Spring clip 34 typicallyexerts about 5 lbs. force on chips 36 and 38, forcing them to maintainsurface to surface contact with ribbon heater wire 32. This represents apressure of about 285 psi.

Note that spring clip 34 is located midway between arms 28 and 30. Thishelps to provide uniform temperature during heating over the length ofchips 36 and 38, as will now be explained. When making a readingelectric current flows through ribbon heater wire 32 causing itstemperature and the temperature of chips 36 and 38 to rise. Arms 28 and30, however, act as heat sinks. Thus, a temperature profile would showlower temperatures in the vicinity of arms 28 and 30 than midway betweenthem. Spring clip 34 located midway between arms 28 and 30 also acts asa heat sink lowering the temperature of chips 36 and 38 at the midwaypoint. Thus, more uniform temperatures are achieved over the lengths ofchips 36 and 38.

Referring to FIGS. 1 and 2, note that chip 36 abuts arm 28 andcompletely masks one side of the currentcarrying portion of ribbonheater wire 32. Similarly, chip 38 abuts arm 30 and completely masks theother side of the current-carrying portion of ribbon heater wire 32.This construction forces light produced at possible hot spots on ribbonheater wire 32 to pass through chips 36 or 38. More importantly, though,it assures that the heat-producing surfaces of ribbon heater wire 32 arein surface to surface contact with chips 36 and 38, thus providinghighly efficient heat conduction therebetwen. As a result, thisconfiguration has about a 0.05 mr infrared equivalent dose.

A high thermal conductivity gas within envelope 20 provides moreefficient heat conduction between chips 36 and 38 and the heat sinksrepresented by clip 34, and arms 28 and 30 to the exterior environment.This helps to avoid incandescence of chips 36 and 38 before the glowpeak is read.

Chips 36 and 38 are so thin that they are almost transparent. This meansthat the thermoluminescent glow produced within the body of each chip 36and 38 is efficiently transmitted optically to the exterior of eachchip. Stated grossly, the glow produced throughout the chip is seen bythe light sensitive detector during readout. Thus, the sensitivity ofthis detector is better by a factor of five than the prior art detectorwhich utilized a mixture of thermoluminescent powder and potassiumsilicate coated onto a heating element.

FIG. illustrates an embodiment alternative to the sub-assembly of FIG.2. In this embodiment heater wire 32' has a circular cross-section, and,as in the foregoing description, is stretched taut while being spotwelded to arms 28 and 30. Chips 36' and 38 have semicircular grooves cutin them so they fit snugly around the cylindrical exterior surface ofheater wire 32' under pressure provided by spring clip 34'.

The configurations illustrated in FIGS. 6 and 7 are very similar inappearance to link fuses and are designed to fit into a clip-in fuseholder in readout apparatus. In both embodiments glass envelope 42 issealed to end cap terminals 44 and 46 by conventional techniques. Inboth cases, envelope 42 encloses the detector assembly either in vacuumor in a high thermal conductivity gas. Both configurations may utilizethe subassembly of FIG. 5 (not shown).

In FIG. 6, electrical leads 48 and 50 extend from end cap terminals 44and 46, respectively, and provide mechanical rigidity for the detectorassembly. Electrical leads 48 and 50 are bent to form arms 52 and 54 asin FIG. 1. Ribbon heater wire 32 is spot welded to arms 52 and 54,again, as in FIG. 1. As illustrated, chips 36 and 38 and spring 34 areassembled as in FIG. 1.

In the embodiment of FIG. 7, electrical leads 56 and 58 extend axiallyfrom end cap terminals 44 and 46. During assembly ribbon heater wire 32is stretched taut and spot welded to electrical leads 56 and S8. Chips36, 38 and spring clip 34 are assembled on ribbon heater wire 32 ashereinbefore described.

In the embodiments of the invention herein disclosed, it has been foundthat no spurious luminescence occurs during measurement of an irradiateddetector. A high thermal conductivity gaseous environment, or anenvironment of carbon dioxide, contributes to the sensitivity of thedetectors described. Moreover, the detectors described are inexpensiveto manufacture and are thoroughly reliable. Through the use of flatchips, the manufacturing rejection rate of thermoluminescent members hasbeen minimized. Further, for manufacturing purposes, the detectorsdescribed can be guaranteed to withstand 1,000 heat cycles withoutdegradation of performance.

Further modifications will occur to those skilled in the art and allsuch are considered to fall within the spirit and scope of the inventionas defined in the appended claims We claim:

I. A radiation detector comprising:

a heater wire;

first and second chips of thermoluminescent material disposed onopposite sides of the heater wire and in surface contact with theexterior surface of the heater wire;

a spring disposed around the chips of thermoluminescent material to holdthem in surface-to-surface contact with the exterior surface of theheater wire and act as heat sink means to more evenly distribute heatalong the length of the chips;

a transparent envelope enclosing the heater wire,

chips and spring; and

electrical leads connected to the ends of the heater wire providingmechanical rigidity and passing through the envelope.

2. A radiation detector as in claim 1, in which the envelope isevacuated to provide a vacuum around the chips.

3. A radiation detector as in claim 1, in which the en velope alsoencloses a gaseous environment of high thermal conductivity.

4. A radiation detector as in claim 1, in which the envelope encloses agaseous environment of carbon dioxide.

5. A radiation detector as in claim 1 in which the envelope encloses agaseous environment of air.

6. A radiation detector as in claim 1 in which the chips are hot-pressedthermoluminescent material selected from fluorides of the groupconsisting of Li Li, and natural lithium.

7. A radiation detector as in claim 1 in which the chips are made ofhot-pressed manganese-activated calcium fluoride.

8. A radiation detector as in claim 1 in which the heater wire has acircular cross section.

9. A radiation detecor as in claim 8 in which the chips havesemicircular grooves cut in them.

10. A radiation detector as in claim 1 in which the heater wire is aribbon heater wire.

11. A radiation detector as in claim 1 wherein the 14. The radiationdetector of claim 1 wherein said heater wire is stretched taut betweenthe electrical spring is located at approximately the midpoint of theleads to prevent it from wrinkling and buckling during length of thechips.

readout. 15. The radiation detector of claim 18 wherein the 12. Aradiation detector as in claim 1 including first 5 heater wire isstretched taut between the electrical and second end caps sealed toopposite ends of the deleads to prevent it from wrinkling and buckling.tector, and wherein said electrical leads comprise a first 16. Theradiation detector of claim 1 including first lead which connects tosaid first end cap and a second and second end caps sealed to oppositeends of the delead which connects to said second end cap. tector, andwherein said electrical leads comprise a first 13. A radiation detectoras in claim 1 wherein said lead which connects to said first end cap anda second electrical leads provide further heat sinks at the ends of leadwhich connects to said second end cap. the heater wire.

Patent No. 3'775'6l4 Dated b r 7, 1973 Inventor(s) Ray Winn and JosephC. Ennis It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

In Column 10, line 4- of the printed patent (patent claim 15) 1 changeto 1L Signed and sealed this 21st d213,: of May 197A.

(SEAL) Atte st:

EDUAHJ PLFLETUt-HER, JR. C MARSHALL BARN Attestingg Officer Commissionerof Patents FORM PO-105O (10-69) USCOMM-DC lO316-P69 u.s. GOVERNMENTrnm'rms ogFlc: IQ! 0-6684,

1. A radiation detector comprising: a heater wire; first and secondchips of thermoluminescent material disposed on opposite sides of theheater wire and in surface contact with the exterior surface of theheater wire; a spring disposed around the chips of thermoluminescentmaterial to hold them in surface-to-surface contact with the exteriorsurface of the heater wire and act as heat sink means to more evenlydistribute heat along the length of the chips; a transparent envelopeenclosing the heater wire, chips and spring; and electrical leadsconnected to the ends of the heater wire providing mechanical rigidityand passing through the envelope.
 2. A radiation detector as in claim 1,in which the envelope is evacuated to provide a vacuum around the chips.3. A radiation detector as in claim 1, in which the envelope alsoencloses a gaseous environment of high thermal conductivity.
 4. Aradiation detector as in claim 1, in which the envelope encloses agaseous environment of carbon dioxide.
 5. A radiation detector as inclaim 1 in which the envelope encloses a gaseous environment of air. 6.A radiation detector as in claim 1 in which the chips are hot-pressedthermoluminescent material selected from fluorides of the groupconsisting of Li6, Li7 and natural lithium.
 7. A radiation detector asin claim 1 in which the chips are made of hot-pressedmanganese-activated calcium fluoride.
 8. A radiation detector as inclaim 1 in which the heater wire has a circular cross section.
 9. Aradiation detecor as in claim 8 in which the chips have semicirculargrooves cut in them.
 10. A radiation detector as in claim 1 in which theheater wire is a ribbon heater wire.
 11. A radiation detector as inclaim 1 wherein the heater wire is stretched taut between the electricalleads to prevent it from wrinkling And buckling during readout.
 12. Aradiation detector as in claim 1 including first and second end capssealed to opposite ends of the detector, and wherein said electricalleads comprise a first lead which connects to said first end cap and asecond lead which connects to said second end cap.
 13. A radiationdetector as in claim 1 wherein said electrical leads provide furtherheat sinks at the ends of the heater wire.
 14. The radiation detector ofclaim 1 wherein said spring is located at approximately the midpoint ofthe length of the chips.
 15. The radiation detector of claim 18 whereinthe heater wire is stretched taut between the electrical leads toprevent it from wrinkling and buckling.
 16. The radiation detector ofclaim 1 including first and second end caps sealed to opposite ends ofthe detector, and wherein said electrical leads comprise a first leadwhich connects to said first end cap and a second lead which connects tosaid second end cap.